This invention generally relates to sensors. More particularly, it relates to a system for remote interrogation of a sensor. Even more particularly, it relates to an improved circuit for low power interrogation.
Smart sensors are being developed for use in roads, bridges, dams, buildings, towers, and vehicles. The sensors may provide many types of information, including displacement, strain, speed, acceleration, temperature, pressure, and force. For remote sensing one challenge has been to provide sensors that consume very low power for reading the sensor and transmitting the data.
For example, a major contributing factor to the deterioration of the nation""s bridges are overloads. These overloads could be caused by heavy trucks or a seismic event. Reliable, inexpensive, and easy to implement sensors that detect and measure the peak loads experienced by a bridge would be useful from a bridge management perspective and as a tool to make the most efficient use of bridge inspection resources
Available sensors have required continuous energizing either for operation or for data transmission, and have required substantial power supplies. For example, a paper, xe2x80x9cMultichannel Strain Gauge Telemetry for Orthopaedic Implants,xe2x80x9d by G. Bergmann, et al., J. Biomechanics Vol. 21 no. 2 pp 169-176, 1988, describes remote powering of a Wheatstone bridge with active strain gauges that require continuous power. A paper, xe2x80x9cRemotely powered, multichannel, microprocessor based telemetry systems for smart implantable devices and smart structures,xe2x80x9d by Chrisopher Townsend, et al, describes an implantable sensor telemetry system that uses low power microprocessors integrated circuits, Wheatstone bridge signal conditioning, and a remote powering system. The Wheatstone bridge has advantage in providing temperature compensation. However, the bridge circuit also requires a continuous voltage and flow of current, so substantial energy is eventually used. Conventional Wheatstone bridge signal conditioners, such as Townsend""s, require instrumentation amplifiers and analog to digital converters which increase the power demand, size, and complexity of these systems.
International patent WO 87/00951 shows an inductive sensor used as the feedback element in an astable multivibrator. This circuit requires a non-differential sensor, which results in poor temperature stability. In addition, the astable multivibrator requires continuous power to operate.
A book, xe2x80x9cCapacitive sensors design and Applications,xe2x80x9d by L. K. Baxter, IEEE Press, 1997, shows a microcontroller providing a train of pulses or a microcontroller providing a single interrogation pulse to excite a capacitive limit switch. However, the circuit described by Baxter does not provide a way to measure more than the two positions of the capacitor and does not compensate for changes in temperature.
A paper, xe2x80x9cMicrominiature, high resolution, linear displacement sensor for peak strain detection in smart structures,xe2x80x9d by Steven W. Arms, et al., proceedings of the SPIE
5th Annual International Conference on Smart Structures and Materials, San Diego, Calif., Mar. 1-5, 1998, describes a differential method of capturing the peak displacement of a member attached to a structure without requiring any power. The paper did not describe micropower methods for remote interrogation.
A paper, xe2x80x9cAn Advanced Strain Level Counter for Monitoring Aircraft Fatiguexe2x80x9d, by D. E. Weiss, Instrument Society of America, ASI 72212, 1972, pages 105-108, describes an inductive strain measurement system which measured and counted strain levels for aircraft fatigue. The system includes an LVDT, signal conditioning, and a data recorder, and required power of 28V.
U.S. Pat. No. 3,798,454 1974 to Weiss describes an optical method for counting, classifying according to magnitude, and recording acceleration signals. The system is resettable, small, and light weight but requires power for operation.
U.S. Pat. No. 5,539,402 to Bozeman described a device capable of electrically recording maximum sensed values, using microfuses. The device converts the analog acceleration signal of an accelerometer to digital levels and connects this digital output to a microfuse memory device. A signal on a particular output line triggers an associated microfuse in the memory device. This system is useful for storing maximum accelerations, and has the advantage of storing information reliably even if power is lost after data has been stored. The system requires battery power to measure and record maximum accelerations.
Fiber optic strain sensors are a popular way to monitor structures, since several very long optical fibers can be used to gather distributed strain and temperature data from many points over very large structures. However, these sensors require expensive equipment for monitoring and acquiring data. They also require special care during installation to prevent fiber breakage during the construction process. Fiber optic connectors, which allow the embedded fibers to egress from the structure in order to be interrogated by external equipment, adds to construction costs and to the difficulty of sensor installation. Furthermore, like previously described systems, they also require power, which can fail at critical moments, such as during a violent storm when important data needs to be collected
Conventional bonded foil strain gages are subject to drift due to delamination of their bond to the structure under test over time, and with exposure to moisture. Considerable surface preparation of the structure, and exotic coating steps are required in order to maintain reasonable short term recordings from a bonded gauge to be exposed to the environment. More stable vibrating wire strain gauges are typically welded to the structure under test, but these welds may cause localized corrosion. Furthermore, neither bonded or vibrating wire strain gauges include a way to passively detect peak displacements or strains.
Sensing systems for civil structures may be powered by batteries and/or solar panels, with multiple sensors requiring long cable runs and connectors. Totally wireless systems eliminate cabling and connector costs, but typically require batteries to be deployed at each sensing node. In order to capture a peak strain event, these batteries must power the sensor continually and may need to sample and log data at high rates. Passive strain sensors do not require power; these can be interrogated(and powered) later to assess damage to load bearing elements.
U.S. Pat. No. 5,086,651 to Westermo and Thompson describes a passive peak strain sensor that uses TRansformation Induced Plasticity (TRIP) steels. However, this sensor cannot be calibrated over its full operating range without irreversible damage to the sensor and the device can only be used oncexe2x80x94since TRIP material yielding is an irreversible effect. In addition the sensor is large in size, has low resolution, and is inherently non-linear.
U.S. Pat. No. 5,914,593 to Arms et al. describes a temperature gradient compensation circuit for two kinds of DVRTs. The first kind, shown in FIG. 4 of the patent, is a core type DVRT which utilizes two coils to detect the change of position of a ferrous or conductive core. The second kind, shown in FIG. 5 of the patent, uses a coil encapsulated within a sensor housing to sense the distance from a conductive or ferrous target. The first kind may touch and the second kind does not touch the target.
Thus, a better system for acquiring and transmitting data is needed that uses less energy and that provides temperature compensation, and this solution is provided by the following invention.
It is therefore an object of the present invention to provide a circuit for reading sense data that directly uses ac power from a remote source;
It is a further object of the present invention to lower power requirements for a sensor by providing a circuit in which power received from a remote source is used directly, without rectification;
It is a further object of the present invention to provide a differential sensor in a Wheatstone bridge configuration with an ac signal from a remote power supply to provide a low power data sensing circuit;
It is a feature of the present invention that the Wheatstone bridge provides for a temperature compensated reading of the differential sensor;
It is a further feature of the present invention that the remotely powered interrogation system provides ac power for running the sensor; and
It is an advantage of the present invention that the circuit for reading a sensor uses very low power.
These and other objects, features, and advantages of the invention are accomplished by an electronic device, comprising a first coil for receiving an electromagnetic signal from a remote source, wherein the electromagnetic signal induces an ac voltage signal in the first coil. The device also includes a circuit comprising a first element and a second element. The first element is changed by movement of a mechanical member. The first element voltage signal derived from the first coil. The second element uses dc voltage obtained from an ac voltage signal derived from the first coil.
Another aspect of the invention is accomplished by an electronic device, comprising a first coil for receiving an electromagnetic signal from a remote source, wherein the electromagnetic signal induces an ac voltage signal in the coil. The device also includes a circuit comprising a first element and a second element; The first element comprising a sensor. The sensor uses an ac voltage signal derived from the first coil. The second element uses dc voltage obtained from an ac voltage signal derived from the first coil.
Another aspect of the invention is accomplished by a method of reading a remote sensing device. The method includes the the step of bringing a reader in range of a remote sensor. The remote sensor comprises a first coil for receiving an electromagnetic signal from the reader. The electromagnetic signal induces an ac voltage signal in the first coil. The remote sensor further comprises a circuit having a first element and a second element. The first element includes a sensor, wherein the sensor uses an ac voltage signal derived from the first coil. The second element uses dc voltage obtained from an ac voltage signal derived from the first coil.
Another aspect of the invention is accomplished by a method of reading a remote sensing device. The method includes the step of bringing a reader in range of a remote sensor. The remote sensor comprises a first coil for receiving an electromagnetic signal from the reader. The electromagnetic signal induces an ac voltage signal in the first coil. The remote sensor further comprises a circuit comprising a first element and a second element. The first element is changed by movement of a mechanical member. The first element uses an ac voltage signal derived from the first coil. The second element uses dc voltage obtained from an ac voltage signal derived from the first coil.